This invention relates generally to means and methods for thermal storage, and more particularly, to two-phase thermal storage means and methods.
Thermal storage systems are useful in many applications, such as for power plant load leveling as described in U.S. Pat. No. 3,848,416(Bundy). In these systems thermal energy is preferably stored by exploiting the mechanism of latent heat storage rather than of sensible heat storage so as to obtain a more compact unit. Such latent heat or "two-phase" thermal storage systems may incorporate either a gas-to-liquid or a liquid-to-solid phase transformation. However, gas-to-liquid phase transformations typically involve large variations in fluid volume, thereby severely complicating the design of an associated thermal storage system.
Liquid-to-solid thermal storage systems also present several design complications including heat transfer coefficient degradation and low volumetric efficiency. For example, in a conventional tube-bundle heat exchanger in which cooling tubes are immersed in a supply of liquid phase-transformation material, the cooling of the material results in the solidification thereof on the tube surfaces. As the solids accumulate on the tubes, the heat transfer coefficient for the unit decreases rapidly, with the heat transfer rate eventually becoming too low for most practical applications. Thus, in an exemplitive application involving a vapor compression cycle device, such a reduction in heat transfer rate would effect a corresponding degradation of the device coefficient of performance until a point is reached at which the continued operation of the device would be impractical. Additionally, when this point is reached, there is typically a core of liquid phase-transformation material remaining in the regions of the heat exchanger unoccupied by the solidified material. This remaining liquid represents a less than optimum system volumetric efficiency since the volume occupied by the liquid could be used for additional thermal storage.
Of course, problems of heat transfer coefficient degradation and low volumetric efficiency are not limited to tube-bundle systems but are present to varying degrees in substantially all conventional liquid-to-solid two-phase thermal storage systems including those which involve the direct contact of two liquids as typified by the system described in the above-noted U.S. Pat. No. 3,848,416(Bundy).
Accordingly, it is an object of the present invention to provide a new and improved two-phase thermal storage means and method.
Another object of the present invention is to provide a new and improved thermal storage device capable of high volumetric efficiency.
Still another object of the present invention is to provide a new and improved two-phase thermal storage means and method which enables a substantially continuous, efficient transfer of heat to and from storage.